Secret communication



April 1, 1947. w. w. HANSEN 2,418,119

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My invention relates to communication systems and concerns particularlymethods and apparatus for conveniently carrying out secret radiotelephone or telegraph communication.

It is an object of my invention to provide an Aimproved secretcommunication system which is `impossible to decipher by an unauthorizedlistener and the intelligence of which can be picked up by neither wideband nor narrow band receivers.

`A further object of my invention is to provide a secret telephonecommunication system in which direct telephone communication may becarried out without delay for decoding.

Other and further objects and advantages will become apparent as thedescription proceeds.

In carrying out my invention in its preferred form, I utilize some formof frequency-modulation or phase-modulation radio carrier frequencytransmission. For transmission of the desired or genuine signal, Iutilize narrow swing modulation, and for masking or confusing thegenuine signal and preventing unauthorized listeners from picking up thesignals, I utilize wide swing modulation. The wide swing modulationcomprises one or more, preferably a plurality of, different lowfrequencies carrying no useful information. In fact, usually an infinitenumber of frequencies or a continuum will be employed. The combinationof wide and narrow swing modulation forms an unintelligible signal.

For the reception of the signals, I provide a frequency modulationreceiver system arranged for selecting the narrow swing modulatedcarrier of the transmitting apparatus. To permit select- `ing thedesired signal, I provide a device for locally producing low frequenciescorresponding to the interference introduced at the transmitter withmeans for producing modulations corresponding thereto. Furthermore, Iprovide apparatus at the receiver for separating such locally producedinterference modulations from the received signal. In this manner, anintelligible signal remains in the frequency modulation receiver of anauthorized listener.

A better understanding of my invention'will be afforded by the followingdetailed description considered in connection with the accompanyingdrawings, and those features of my invention which are believed to benovel and patentable will be pointed out in the claims appended hereto.

In the drawings,

Fig. 1 is a schematic or block diagram of a secret radio communicationsystem constituting one embodiment of my invention, and

Fig. 2' is a block diagram of a modified system.

5 Claims. (Cl. Z50-6) In the specific embodiment of my inventionrepresented for the sake of illustration in Fig. l, I providetransmitting apparatus collectively designated by the reference numeralI I and receiving apparatus collectively designated by the referencenumeral I2. The transmitting apparatus II-comprises a relativelynarrow-swing frequency modulation transmitter or generator I3, means formodulating the transmitter I3 in accordance with a genuine signal to betransmitted, for example, a microphone I4 arranged Vto be spoken into bythe authorized communication oflcer, an antenna or radiator I5 forradiating a signal, and means for superimposing on the radiated signalwide swing modulations, in which the modulations convey no usefulinformation. Such means for superimposing wide swing modulations or-interference comprise a suitable source of noise IB, an amplifier I1for building up audio frequency voltages corresponding to such noisesufficiently for modulation of a carrier, a wide swing frequencymodulation transmitter or generator `I8 made responsive to the output ofthe `amplifier I'I and a mixer, for example, an additive mixer I 9 forcombining the narrow swing modulated signal of the transmtter I3 and thewide swing modulated interference signal of the transmitter 1.8 in thewaves radiated from Ythe antenna I5. It will be understood `that thegenerators I3 and 'I8 need not have high power output as the power forVtransmission may be supplied by providing 'a final amplifier (notshown), coupled to theantenna I5 and excited by the output of the mixerFor the purpose of making it more difficult, in fact,`virtuallyimpossible for unauthorized listeners to decipher the signals, Ipreferably interpose a multi-channel selectively adjustable band passaudio'lter 2G between the output of the amplifier il and the input tothe wide swing frequency modulation transmitter I8. For reasons whichwill become moreapparent hereinafter, I also 'prefer to interposebetween the amplifier I1 and the wide swing frequency modulationtransmitter I8 another transmitter `2I for converting the noise outputof the amplifier I'I into a modulated carrier and a modulation receiver22 for reconverting the modulated radio frequency output of thetransmitter 2l into audio frequency noise correspending to the output ofthe source II.` Thefrequency transmitter 2I is provided with an antennaradiator 23 for radiating the major portion of its energy to a receivingstation, only a portion of the output of the transmitter 2| being fed tothe receiver 22. The transmitter 2| and the receiver 22 are designed forthe same type of modulation. They may be frequency-modulation devices,but not necessarily so. Amplitude modulation may also be employed.

The noise source I6 may be a suitable source of noise such as a resistoror a saturated diode producing thermal or shot eifect noise which may beamplified by a suitable device such as the amplifier I1. Preferably thenoise source includes frequencies in the genuine signal in order to makeltering out of noise impossible.

For the sake of illustration and more conveniently distinguishingbetween the elements of the apparatus, I have arbitrarily designatedcertain frequencies and modulation swings for various portions of theapparatus, but it will be understood that my invention is not limited tothe specific values mentioned for the sake of illustration. Thetransmitters 2I and I8 are preferably adjusted for operation atfrequencies sufiiciently different notto cause interference if they arevboth of 'the' frequency modulation type. For example, the transmitter 2Imay have a 45 megacycle carrier and the transmitter I8 may have a 30megacycle carrier. The transmitter I3 may be adjusted for anothercarrier frequency from the transmitter I8, for example, 40 megacycles.The swing of the transmitter I8 is made considerably greater than thatof transmitter I3, for example, the swing of transmitter I8 may be 1`meg'acycle whereas that of transmitter I3 is only 1()` kilocycles. Thereceiver 22 is of course tuned to the frequency of the transmitter 2|.'Ihe transmitter 2| need not necessarily be a1- ranged for either a'wide swing or a narrow swing if `it is designed for frequency modulationas either type of transmitter may satisfactorily be employed, as well asan amplitude modulated transmitter.

The `multi-channel lter 20 may be of any suitable type providing aplurality of dierent audio frequency channels which may or may notoverlap as preferred.` It may take the form simply of an audio amplierwith base and treble tone control. 4For subjecting an unauthorized-listener to the utmost diiiculty, ten or twenty channels at least maybe provided, and means may be provided for selecting several differentchannels for simultaneous use. Although more Achannels are preferablyemployed, in order to I simplify the drawing, I have schematicallyrepresented four diiferent audio frequency channels 24,25, 26 and 2l.For selectively connecting or disconnecting the channels, there areprovided pairs of switches 28, 29, 30 and 3I. As shown in the drawing,the switches 28, 2S, and 3l are open and the switches 3E! are closed sothat only the channel 26 is in use. However, in the actual apparatusWhere a greater number of channels is preferably employed, I prefer tohave the switches in the several, for example, four or v channels closedsimultaneously.

`A'I'he mixer I9 may be of any suitable type and ,does not in itselfconstitute a part of my present invention'but may, for example, besimilar in `principle of operation to the mixers utilized insuperheterodyne radio receivers. One form of such mixers comprises amulti-grid vacuum tube having one signal supplied to one grid and thesecond signal supplied to another grid. If the mixer I9 is an additivemixer, it may be followed bya suitable device such as a high-pass iilterI9a for cutting off the lower sidebands and the transmitter carriers andpermitting only the upper sidebands, resulting from the combination ofthe of the mixer 23.

outputs of the transmitters I3 and I3 to be radiated from the antennaI5. For the transmitter frequencies assumed, the filter ISa, would outoif the 30 and 40-megacycle carriers, and the approximated IO-megacyclelower sidebands, retaining only the sidebands centered on megacycles,which may be called the radiated or summation carrier.

As an added precaution in preventing unauthorized listeners fromattempting to receive the output of the narrow swing transmitter i3 witha wide band receiver, I prefer to provide a second noise source 31?which is arranged to combine radio-frequency noise with the output ofthe mixer I9. The noise source 35i is designed to produce a wideradio-frequency spectrum, e. one megacycle wide. It may be strong incomparison with the signal produced by the modulations of the signalsource I4, if the swing of the F. M. generator I3 is small compared tothe spectrum width of the generator I3 and of the noise source 34.

The receiver I2 is largely a counterpart of the transmitter I I. Itincludes a frequency modulation receiver 35 designed for sharp tuning ornarrow band reception tuned to the output of the gennine-signalnarrow-swing transmitter I3, a receiving antenna 35, and apparatuscollectively designated by the reference numeral 3l for removing fromthe signal picked up by the antenna 36, the wide swing interferencemodulations superimposed upon the radiated signal by the wide swingfrequency modulation-transmitter I3. The receiver 35 includes headphones38 or the like for converting the electrical signal into sound waveswhich carry the desired information to the receiving oiiicer.

The apparatus 3l includes means for reproducing audio frequencyinterference identical with that passed by the filter 2t of thetransmitter, converting such interference into radio frequencymodulations and subtracting such modulations from the waves picked up bythe receiving antenna 36. As illustrated in the drawings, the apparatus31 comprises a receiver 32 with a receiving antenna llt! tuned andsuitably arranged to receive the radiations from the general noisetransmitting antenna 23, a multi-channel selectively adjustable bandpassaudio lter tI, a frequency modulation transmitter or generator 42 and asubtractive mixer 13. For adjusting the swing of the F. M. transmitteri2 to that of the 'transmitter I8, a gain-control or variable attenuator42 is provided.

For providing a basis of reference in order to reproduce accurately theselected audio noise superimposed at the transmitter, the general noisebefore filtering is transmitted from the antenna 23 to the antenna il asrepresented by the dotted arrow IVI. This noise is picked up by thereceiver 39 converted into audio frequency noise, ltered by the lter 4I,supplied to the transmitter 42 for reconversion into radio frequencymodulations, and passed in to one side The receiving antenna 36 is alsoconnected toV the mixer d3 so that the output of the mixer A3, suppliedthrough the conductors l5 represents the difference between theradiations received on the antenna 36 and the selectively produced noisemodulations supplied by the transmitter 42.

It will be understood that the lter III corresponds to the filter 29 ofthe transmitter and that corresponding switches therein (not shown) areclosed so that the same channels of audio 5 frequency noise are passedin the filter 2li and the lter 4I.

The transmitter 42 corresponds to transmitter I8 in `carrier frequencyand possible range of swing.

If the mixer Ie is an additive mixer, the mixer 43 should be asubtractive mixer and vice versa. Thus, if the mixer is is additive andthe lower sidebands have been out off from the waves received by theantenna and transmitted to the receiver 35, the mixer 63 should besubtractive, and its transmission characteristic be such as to cut offthe received carrier and upper sidebands retaining only the lowersidebands centered on 40 megacycles, in the case of the frequenciesassumed in the transmitter. A filter 43a may be provided such that itcuts off above a predetermined frequency, in this case, aboveapproximately 40 megacycles.

It will be apparent from a consideration of the characteristics of theapparatus that an unauthorized listener not having a duplicate of thereceiving apparatus I2 will be unable to receive the genuine signalswhether he uses a wide band or a narrow band receiver. A wide bandreceiver will pick up very strongly the unintelligible noise output ofthe transmitter IE and will also pick up the noise output from thesource 34 if such a source is provided. On the other hand, if theunauthorized listener utilizes a narrow band receiver in an attempt totune in the signal and tune out the interference, the wide bandmodulations of the transmitter i8 will prevent proper operation of thereceiver. Such wide swing modulations in effect tune the transmited waveoutside the frequency band of the unauthorized listeners receiver forthe greater portion of the time and thus make such a receiverinoperative.

The effect of the two noise sources It and 34 may be appreciated byconsidering hypothetical simplifications. It may be assumed first thatthe noise source td is disconnected and that the noise is actually asingle frequency f1 which produces a frequency swing iyz in theeffective carrier or summation carrier radiated from the -f radiator I5,and indicated for the sake of illustration as having a frequency of 70megacycles per second.

Then if' there is no signal supplied from the source I 4, the spectrumof the radiated carrier has components at frequencies infr away from thecarrier, where n is any integer. The amplitudes fluctuate erratieallywith n until nfl approaches a magnitude of the order of f2 after whichthey decrease rapidly. (This assumes that f1 is small in comparison withf2, and it will presently appear that this corresponds to correctoperating conditions.) This spectrum could be measured with'an amplitudemodulated receiver provided its pass band is less than f1. The processwill be made difficult if f1 is small. A frequency modulation receiverwould simply have an output of f1. With this assumed simplification,relatively little secrecy is obtained.

Let it be assumed now that a signal of a single frequency f3 isintroduced from the source I4. Then each sideband nfl is split byamounts mfs, where m is any integer. This enormously complicates thespectrum and increases the difculty of amplitude analysis. |`Ehe problemcan be made insoluble by replacing fi or f3 by a continuous spectrum.Therefore, ji is made continuous, because f3 must conform to the inputsignal. Thus, f1 should be a noise voltage, preferably with if f1 ismuch stronger -than ,f3 and if it is close to f3; that is, the frequencyswing due to f1, the noise should be large compared to f3, the signal.Furthermore, f1 and f3 should be close together. This verifies theassumption above that f1 is small in comparison with f2; because thesignal swing should be o f the order of magnitude of f3 to get goodmodulation, fr is of the order of f3, and f3 is small in comparison withf2. Moreoven if f1 is replaced by a voltage with a continuous spectrum,the receiver output will be noise plus signal with the noise moreintense than the signal, in the ratio of Ylf2/f3. If -this is large andthe noise frequency includes f3, filtering in the receiver output willbe impossible.

If, therefore, the noise has important components throughout the rangeof signal frequencies and if the swing due to the noise is largecompared to that due to the signal, deciphering is impossible. This istrue even without the noise added from the source 34.

The effect of the noise source 3d may now be considered.

If it is again assumed that f1 is a single frequency and there is nosignal from the source Hi, then a spectrum of the radiated output fromthe antenna I5 is a super-position of a continuous spectrum and a linespectrum. This can theoretically be analyzed by an amplitude modulationreceiver but only with considerably enhanced difficulty because itsbands must be made narrower than f1 to out out noise. Moreover, afrequency modulation receiver, which in the first simplification gavesimple results, will now give noise plus f1, and if the noise is intenseenough, there will be difficulty in finding f1.

It may be concluded that if averaged over the band f2, the noise isconsiderably larger than the signal; the relatively simple system withf1 a single frequency can be solved by an unauthorized listener onlywith difficulty.

If f1 is made continuous, the problem of intercepting transmittedinformation is greatly increased.

The noise supplied by the source 34 may be large compared with thesignal of frequency f2 of the source Ill without causing dimculty withthe authorized receiver. This is true because the authorized receiverneed only take in a band width f3, whereas an unauthorized listener musttake in the band f2, and. both will get the same signal energy. Thisshows again the desirability or" making f2 large in comparison with f1and f3, for the ratio determines the permissible amount of additionalnoise which may be supplied by the source 34.

In order to minimize diiculty in the apparatus of Fig. l resulting fromany possible distortion in the receivers 22 and 39, it is desirable thatthese receivers be identical in type and construction. Under thesecircumstances there is no need for the receivers to be distcrtionlessbecause they will produce like effects in the energy supplied to thefilters Eil and lll.

However, if the transmitter 2! and the receiver 39 are madesubstantially distortionless or are so constructed as to minimizepossibile diiculty from distortion, the system of Fig. l may besimplified by omitting the receiver 22.

VIn this case the noise output of the source I6 is supplied directly tothe input side of the filter 2li. In such a case the noise source I6 hasparallel outputs to the noise transmitter 2l and the filter 2B, asindicated schematically in Fig. 2.

Fig. 2 illustrates an even further simplification in the apparatus ofFig. 1 which may be employed if care is taken in the design andadjustment of the apparatus. In this case, two separate frequencymodulation generators I3 and I8 are not required. Instead, a singlefrequency modulation generator I8 may be employed with parallelmodulation sources. from the lter 20 representing selected noise and thedevice IB representing the genuine signal. If energy is to be radiatedfrom the antenna I5 with a 'lO-megacycle effective carrier, thegenerator I8 is designed to produce a carrier of '70 megacycles.

The generator I8' is so constructed as to be capable of wide swingfrequency modulation, and the output level of the lter 20 is maintainedat a value such as to cause Wide swing modulation of the generator I8',for example, a swing of about one megacycle, to correspond with theapparatus of Fig. 1. The signal source I4, on the other hand, is sodesigned as to produce a normal modulation level such as to producenarrow swing modulation of the generator I8', for eX- ainple, alO-kilocycle swing corresponding to the apparatus of Fig. l. In thiscase, the antenna I5 will radiate the carrier modulated with noise overa wide swing and with the desired signal over a narrow swing, as in thecase of the apparatus of Fig. l.

It will be understood that in the apparatus of Figs. 1 or 2 an amplifier52 may be interposed in the connections to the antenna I5. If theadditional noise source 34 of Fig. l is employed in the apparatus ofFig. 2, it may be connected to supply noise in parallel with the outputof the generator I8 to the input side of the amplifier 52, as indicatedschematically.

For the modified embodiment of Fig. 2, the same receiving system 3l asshown in Fig. 1 may be employed. Since the wide swing noise supplied bythe filter 28 to the frequency modulation generator I8 may be removedfrom the radiated carrier by the frequency modulation generator to thereceiver, the apparatus of Fig. 2 will operate in the manner describedin connection with Fig. l. It will be understood that more carefuldesign and adjustments of the apparatus are required than in the case ofthe apparatus of Fig. l because, to a considerable extent, the use ofidentical units in the transmitting and receiving station of Fig. lcauses distortions introduced in the transmitting station to be removedagain in the receiving station.

The apparatus illustrated is particularly useful for communicationbetween ships of a fleet, for example, where orders and information mustbe transmitted and understood immediately for proper carrying out ofmaneuvers and no time can be allowed for decoding messages sent in codesufficiently complicated to avoid prompt decoding by unauthorized orundesired listeners. With such use of the apparatus, it will beunderstood that the particular channels of the filters 20 and 4I whichare to be used will be designated in previously given orders, or thechannels may be changed for different times of the day. With as many astwenty different channels to choose from, it will be apparent that atremendous number of permutations and combinations is possible, so thatin effect each of these different combinations serves as a differentautomatically operating coding and decoding system for secretcommunication.

It is impossible for an unauthorized listener to decipher the codebecause unlike other types of codes, the system cannot be deciphered byworking on one unknown element at a time until after with infinitepatience, if necessary, all elements have been deciphered. In this case,the code can be deciphered only by simultaneously knowing every factorof the code even if the unauthorized listener should have succeeded inobtaining a duplicate of the receiving apparatus I2.

However, if desired to make it still more difficult for an unauthorizedlistener to receive the signals, synchronized interchanging switches maybe provided for the filters 20 and Il for progressively changing theconnections of the switches 28 to 3|, for example, in the transmitterand corresponding switches in the receiver. For example, a shipschronometer 46 may be connected through suitable linkage represented bydotted line 4l to operate the selection switches 28 to 3l in accordancewith a sequence or progression fixed by cams (not shown) and a shipschronometer i8 at the receiving station may be arranged to drivecorresponding linkage i9 for operating similar changeover switches (notshown) in the filter 6H.

Since many changes could be made in the above construction and manyapparently Widely difierent embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or Shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A system of secret radio communication, comprising transmitting andreceiving apparatus, said transmitting apparatus comprising a relativelynarrow swing frequency modulation transmitter, means for modulating saidtransmitter with a genuine signal to be received by an authorizedlistener, a wide swing frequency modulation transmitter, a source ofaudio frequency noise, a lter for selecting predetermined components ofsaid noise source and connections between said lter and said wide swingfrequency modulation transmitter for modulating the latter transmitterwith such selected audio frequency noise components, a noise transmitteroperating on a diiierent frequency with input connections from saidnoise source, an output antenna for transmitting a radio frequency wavemodulated in 'accordance with such noise, a second output antenna, andmeans for combining the outputs of the rst and second transmitters inthe radiation from said second antenna, said receiving apparatuscomprising a noise receiving antenna and a frequency receiver tuned tosaid noise transmitter, a third frequency modulation transmitter and afilter interposed between said receiver and said third transmitter forpassing the saine noise components as said rst filter, a signalreceiving frequency modulation receiver tuned to the signal producingnarrow-swing transmitter of the transmitting apparatus, a secondreceiving antenna responsive to the radiation from said secondtransmitting antenna, and means for subtracting the output of said thirdfrequency modulation transmitter from the energy received by said secondreceiving antenna for supplying the difference to said signal-receivingreceiver.

2. A system of secret radio communication comprising the elements setforth in claim 1 to- 9 gether with a second noise source arranged forcombining its output with the signal inputs supplied to the secondantenna.

3. A communication system comprising a radio transmitter and a radioreceiver; said transmitter comprising means for generating a radiofrequency carrier, and means coupled to said transmitter for frequencymodulating said carrier in accordance with a combination of a desiredintelligence signal and an interference signal; and said receivercomprising a local oscillator, a mixer coupled to said local oscillatorand arranged to receive said frequency modulated carrier and to producea heterodyne output signal of frequency varying in accordance with acombination of the frequencies of said frequency modulated carrier andsaid local oscillator, and means for frequency modulating said localoscillator in accordance with said interference signal, whereby thefrequency of said heterodyne output signal varies in accordance withsaid desired intelligence signal.

4. Apparatus for receiving a carrier frequency modulated in accordancewith the sum of a first modulation component and a second modulationcomponent, comprising a receiver local oscillator, a mixer coupled tosaid local oscillator and adapted to receive said frequency modulatedcarrier and to produce a heterodyne output signal of a frequency varyingjointly in accordance with the frequency of said carrier and thefrequency of the output of said local oscillator, means for frequencymodulating said local oscillator in accordance with a wave of thefrequency and amplitude of said rst modulation component, and fre- 5. Acommunication system comprising means for generating a first radiofrequency carrier, means for modulating said first radio frequencycarrier in accordance with a first signal voltage, means for generatinga second radio frequency carrier, means coupled thereto for frequencymodulating said second carrier in accordance with a combination of saidfirst signal voltage and a second signal voltage, a first receiver tunedto said first radio frequency carrier and adapted to reproduce adetected Version of said first signal voltage, a superheterodynefrequency modulation receiver tuned to receive said second radiofrequency carrier, said superheterodyne receiver including a localoscillator, and means coupled to said local oscillator and to said firstreceiver for frequency modulating said local oscillator in accordancewith said detected version of said rst signal voltage.

WILLIAM W. HANSEN.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,480,217 Mills Jan. 8, 19241,612,285 Hammond Dec. 28, 1926 2,272,999 Curtis Feb. 10, 1942 2,207,620Hilferty July 9, 1940 2,204,050 Purington June 11, 1940 FOREIGN PATENTSNumber Country Date 288,714 British Apr. 12, 1928

